Yahya related to me from Malik from Abu Hazim ibn Dinar that Sahlibn Sad said, "People used to be ordered to place their right hands ontheir left forearms in the prayer."Abu Hazim added, "I knowfor sure that Sahl traces that back to the Prophet, may Allah blesshim and grant him peace."
USC-MSA web (English) reference: Book 9, Hadith 50
Arabic reference: Book 9, Hadith 381